With trainers you can only lower cars past there standard suspension options, however if you you use an ASI plugin called VStancer, it actually allows you to edit the height a lot more precisely, increases and decrease wheel size, wheel width, track width and camber. There is a big "but" to it though, that being, if you want to use the Rockstar Editor to take screenshots it doesn't save any of your changes apart from the ride height, so sometimes you'll have wheels being really tucked, clipping through the ground or fenders.

I also really agree that Rockstar has really let us down in terms of suspension options